In this article we investigate a teaching strategy that is similar to that proposed in 1987 by Millar, where we analyze whether students obtain more accurate experimental data or not when submitted to the strategy. The working hypothesis is that if students have previous knowledge of the value they are supposed to obtain out of an experiment, they get more accurate results since they go through it more cautiously and attentively; when their results deviate from the expected value, they retrace their steps, something which does not happen in case they do not know the values beforehand. We corroborate our hypothesis through a statistical test (Student's t-test) applied to two sets of high-school students (experimental and control groups) in a real classroom situation, during experimental classes. We present and consider the consequences of our results when compared to the use of investigative didactic strategies based on hypotheses testing or theory application.